1. Field of the Invention
This invention relates to a method of reading out a radiation image stored in a stimulable phosphor sheet. This invention particularly relates to a radiation image read-out method wherein light emitted by the stimulable phosphor sheet upon stimulation thereof in proportion to the radiation energy stored is detected accurately.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy.
FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the apparatus of FIG. 1, a laser beam 1a of a predetermined intensity is emitted as stimulating rays from a laser beam source 1 to a galvanometer mirror 2. The laser beam 1a is deflected by the galvanometer mirror 2 to form a laser beam 1b impinging upon a stimulable phosphor sheet 3 positioned below the galvanometer mirror 2 so that the sheet 3 is scanned by the laser beam 1b in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam 1b impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1b. As the stimulable phosphor sheet 3 is scanned by the laser beam 1b, the portion of the sheet 3 exposed to the laser beam 1b emits light having an intensity proportional to the radiation energy stored. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its front end face 4a positioned close to the sheet 3 in parallel to the main scanning line. The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its front end face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or stored in a magnetic tape 8, or directly reproduced as a hard copy on a photographic material or the like.
In this manner, the radiation image stored in the stimulable phosphor sheet 3 is read out. However, since the front end face 4a of the light guide member 4 extends approximately over the entire width of the stimulable phosphor sheet 3 in parallel to the main scan line thereon, all light emitted from the portions of the stimulable phosphor sheet 3 covered by the front end face 4a enters the light guide member 4 from the front end face 4a and is detected by the photomultiplier 5. That is, not only the light emitted from the portion of the stimulable phosphor sheet 3 upon which the laser beam 1b impinges at any given instant, in proportion to the radiation energy stored in that portion, but also the other light emitted as described below from the portions of the sheet 3 covered by the front end face 4a enter the light guide member 4 and are detected by the photomultiplier 5. The light other than the light emitted from the portion of the stimulable phosphor sheet 3 upon which the laser beam 1b impinges currently in proportion to the radiation energy stored in that portion embraces after-glows emitted by the stimulable phosphor sheet 3. The after-glows are divided into an instantaneous light emission after-glow and a stimulated light emission after-glow.
By "instantaneous light emission after-glow" is meant the after-glow of light instantaneously emitted by a stimulable phosphor sheet when the sheet is exposed to a radiation to have a radiation image stored in the sheet, the after-glow continuing to be emitted by the sheet while the light intensity decays after the exposure of the sheet to the radiation is ceased. The characteristics of the instantaneous light emission after-glow are generally as shown in FIG. 2, though they will differ depending on the type of the stimulable phosphor constituting the stimulable phosphor sheet. In the graph of FIG. 2, the ordinate represents the intensity of light emission and the abscissa represents time (t). As shown in FIG. 2, when the exposure of a stimulable phosphor sheet to a radiation is ceased after the sheet is exposed to the radiation for a period of .DELTA.t2 from a time t1 to a time t2, the intensity of light instantaneously emitted by the sheet at a light emission intensity A does not immediately decreases to zero, but instead an instantaneous light emission after-glow continues while the intensity thereof decreases along an exponential function curve the time constant of which increases gradually.
For example, decay of the light emission intensity of the instantaneous light emission after-glow is such that a light emission intensity B of the instantaneous light emission after-glow at a time t3 approximately 180 seconds after the exposure of a stimulable phosphor sheet to a radiation is ceased (i.e. t3-t2=180 seconds) is approximately 10.sup.-4 times the intensity of light emitted by the sheet when the sheet is exposed to stimulating rays.
Accordingly, in the case where a predetermined time elapses from when a stimulable phosphor sheet is exposed to a radiation passing through an object to have a radiation image stored in the sheet to when read-out of the radiation image stored is conducted, the intensity of the instantaneous light emission after-glow decreases sufficiently and the effect of the after-glow becomes negligible in the read-out step. However, when read-out of the radiation image is conducted immediately after the radiation image is stored in the stimulable phosphor sheet, for example when a built-in type radiation image recording and reproducing system wherein an image recording section and an image read-out section are installed integrally to record and read out many radiation images continuously and quickly is employed, the light emission intensity of the instantaneous light emission afterglow does not decay sufficiently before image read-out is conducted. As a result, the instantaneous light emission after-glow is detected together with the light emitted by the stimulable phosphor sheet in proportion to the radiation energy stored when the sheet is exposed to stimulating rays, and the effect of the instantaneous light emission after-glow on the electric image signals obtained thereby becomes large.
Further, the light emission by the stimulable phosphor sheet upon stimulation thereof by stimulating rays arises from a portion having a very small area upon which the stimulating rays impinge, whereas the instantaneous light emission after-glow is emitted from the whole surface of the stimulable phosphor sheet exposed to a radiation. Therefore, as the stimulable phosphor sheet 3 is scanned point by point by the laser beam 1b as shown in FIG. 1, the light emitted from a portion of the sheet 3 upon which the laser beam 1b impinges momentarily in proportion to the radiation energy stored in that portion and the instantaneous light emission after-glow emitted from all of the portions covered by the front end face 4a of the light guide member 4 simultaneously enter the light guide member 4 from the front end face 4a and are guided to the photomultiplier 5. In this case, since the area of the portions covered by the front end face 4a of the light guide member 4 is markedly larger than the area of the portion of the stimulable phosphor sheet 3 upon which the laser beam 1b impinges momentarily, the amount of the instantaneous light emission after-glow guided to the photomultiplier 5 becomes not negligible even though a predetermined time elapses after the exposure of the stimulable phosphor sheet 3 to a radiation is ceased and the intensity of the instantaneous light emission after-glow becomes negligibly low as compared with the intensity of the light emitted by the sheet 3 upon stimulation thereof.
By "stimulated light emission after-glow" is meant the after-glow of light emitted by a stimulable phosphor sheet carrying a radiation image stored therein when the sheet is exposed to stimulating rays (e.g. a laser beam) for reading out the radiation image, the after-glow continuing to be emitted by the sheet while the light intensity decays after the exposure of the sheet to the stimulating rays is ceased. The characteristics of the stimulated light emission after-glow are generally as shown in FIG. 3, though they will differ depending on the type of the stimulable phosphor constituting the stimulable phosphor sheet. In the graph of FIG. 3, the ordinate represents the intensity of light emission and the abscissa represents the time (t). As shown in FIG. 3, when the exposure of a stimulable phosphor sheet to stimulating rays is ceased after the sheet is exposed to the stimulating rays for a period of .DELTA.t5 from a time t4 to a time t5, the intensity of light emitted by the sheet upon stimulation thereof at a light emission intensity C does not immediately decreases to zero, but instead a stimulated light emission after-glow continues while the intensity thereof decreases along an exponential function curve with the time constant thereof increasing gradually. (That is, the light intensity decreases rapidly at the beginning and thereafter the rate of decrease in the light intensity becomes gradually lower.)
For example, decay of the light emission intensity of the stimulated light emission after-glow is such that the initial time constant is approximately one microsecond, i.e. the time t6-t5 required for the light emission intensity to become 1/e (D/C=1/e) is approximately one microsecond. In general, since the speed of scanning (in the main scanning direction) of a stimulable phosphor sheet by stimulating rays by use of a galvanometer mirror is approximately 50Hz, it takes approximately 20,000 microseconds for scanning one time. Accordingly, the intensity of the stimulated light emission after-glow decaying along an exponential function curve with the initial time constant of one microsecond becomes very low as compared with the intensity of the light emitted by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to the stimulating rays. Thus the intensity of the stimulated light emission after-glow at each point of the stimulable phosphor sheet becomes almost negligible.
However, the light emission by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to stimulating rays arises from a portion having a very small area upon which the stimulating rays impinge, whereas the stimulated light emission after-glow is emitted from the whole surface of the stimulable phosphor sheet scanned by the stimulating rays. Therefore, as the stimulable phosphor sheet 3 is scanned point by point by the laser beam 1b as shown in FIG. 1, the light emitted from a portion of the sheet 3 upon which the laser beam 1b impinges momentarily in proportion to the radiation energy stored in that portion and the stimulated light emission after-glow emitted from all of the portions covered by the front end face 4a of the light guide member 4 simultaneously enter the light guide member 4 from the front end face 4a and are guided to the photomultiplier 5. In this case, since the area of the portions covered by the front end face 4a of the light guide member 4 is markedly larger than the area of the portion of the stimulable phosphor sheet 3 which is momentarily exposed to the laser beam 1b and which emits light upon stimulation by the laser beam 1b, the amount of the stimulated light emission after-glow guided to the photomultiplier 5 becomes not negligible even though the intensity of the stimulated light emission after-glow becomes negligibly low as compared with the intensity of the light emitted by the sheet 3 upon stimulation thereof.
The after-glows detected together with the light emitted by the stimulable phosphor sheet upon stimulation thereof by stimulating rays as described above constitutes a noise component in the electric image signals obtained by the read-out of a radiation image and make it difficult to accurately read out the radiation image.
The instantaneous light emission after-glow presents a problem particularly when image read-out is carried out immediately after a stimulable phosphor sheet is exposed to a radiation to have the radiation image stored therein. On the other hand, the stimulated light emission after-glow presents a problem particularly when the scanning speed of stimulating rays on the stimulable phosphor sheet carrying the radiation image stored therein is increased.
The effects of the after-glows on the amount of light detected by image read-out will hereinbelow be described in more detail with reference to FIGS. 4A and 4B. FIG. 4A shows a stimulable phosphor sheet 3a carrying a radiation image of the head of a human body stored therein. FIG. 4B shows a graph wherein the abscissa represents the scanning point along the line a on the stimulable phosphor sheet 3a of FIG. 4A and the ordinate represents the amount of light transmitted to a photomultiplier via a light guide member when the stimulable phosphor sheet 3a is scanned by stimulating rays (laser beam) along the line a. In FIG. 4B, the broken line l1 designates the amount of light actually transmitted to the photomultiplier, and the solid line l2 designates the amount of light emitted by the stimulable phosphor sheet 3a upon stimulation thereof when the sheet 3a is exposed to the stimulating rays. The chain line l3 designates the amount of after-glows (i.e. the sum of the instantaneous light emission after-glow and the stimulated light emission after-glow). That is, the sum of the amount l3 of the after-glows and the amount l2 of the light emitted by the stimulable phosphor sheet 3a upon stimulation thereof when the sheet 3a is exposed to the stimulating rays is equal to the light amount l1 transmitted to the photomultiplier. The light amount l1 is converted to an electric image signal by the photomultiplier and then logarithmically converted to reproduce a visible image by use of the logarithmically converted signal. In this case, the signal level obtained when the light amount l1 transmitted to the photomultiplier is converted to an electric image signal and then logarithmically converted is different from the signal level obtained when only the amount l2 of light emitted by the stimulable phosphor sheet 3a upon stimulation thereof by the stimulating rays is converted to an electric image signal and then logarithmically converted. Therefore, when a visible image is reproduced by use of the image signal obtained by converting the light amount l1 transmitted to the photomultiplier, the visible image thus reproduced becomes different from the correct image. That is, the visible image reproduced becomes incorrect or unsharp, and a very real problem arises with regard to the image quality, particularly diagnostic efficiency and accuracy.